With the increasing cost and potential restrictions on availability of conventional fuels such as petroleum, natural gas, etc., as sources of energy, considerable research effort has been directed towards developing other sources of energy. One area which has been considered as an important alternative source of energy is solar energy. In most attempts to make use of the sun's energy, the main research effort has been directed toward developing inexpensive, large capacity devices known as solar cells which directly transform the sun's energy which initially is in the form of photons into electricity.
It is estimated that if part of the petroleum required to operate electric power plants in a country such as the United States could be replaced by solar energy, an approximate saving of about two million barrels of oil per day could be saved. In order to meet such an objective, solar power generation must be capable of producing 40 GW of electricity on a continuous basis. Assuming that solar is in general available about 20% of the day, 40 GW of continuous photovoltaic power would require 200 GW of solar array capacity, including a proper storage system.
In order to arrive at a cost which would justify changing petroleum-energy electric power plants to solar power energy electric power plants, a method of producing ultra pure silicon must be devised at a cost capable of competing with presently available sources of energy.
Presently, available procedures involve the use of chemical silicon vapor deposition on a bed of silicon seed particles (see U.S. Pat. No. 4,207,360), or the conversion of low grade silicon to high purity silicon by forming silicon tetraiodide. In general, such a starting silicon has a purity of about 99.8%. It is obvious that any procedure starting with silicon having such a high degree of purity will cause the obtained silicon to be very expensive.
Accordingly, it would appear highly desirable to provide a method for preparing ultra pure silicon from the unexpensive and readily available metallurgical grade silicon.